This invention relates to reactive multilayer foils, and, in particular, to a method of making such foils using plastic deformation.
Reactive multilayer coatings are useful in a wide variety of applications requiring the generation of intense, controlled amounts of heat in a planar region. Such structures conventionally comprise a succession of substrate-supported coatings that, upon appropriate excitation, undergo an exothermic chemical reaction that spreads across the area covered by the layers generating precisely controlled amounts of heat. While we will describe these reactive coatings primarily as sources of heat for welding, soldering or brazing, they can also be used in other applications requiring controlled local generation of heat such as propulsion and ignition.
In almost every industry, improvements in bonding are becoming increasingly important as technology advances. This is especially true as the bodies to be bonded become smaller and more fragile. Additionally, new materials are often difficult to bond and have presented many problems to industry.
Many methods of bonding require a heat source. The heat source may be external or internal to the structure to be joined. An external source is typically a furnace that heats the entire unit to be bonded, including the bodies (bulk materials) to be joined and the joining material. An external heat source often presents problems because the bulk materials can be sensitive to the high temperatures required for joining. They can also be damaged by the mismatch in thermal contractions.
Internal heat sources often take the form of reactive powder. Reactive powders are typically mixtures of metals or compounds that will react exothermically to form a final compound or alloy. Such powders, developed in the early 1960s, fostered bonding by Self-Propagating, High-Temperature Synthesis (SHS). However, the energy released and the diffusion of the energy are often difficult to control in SHS reactions. As a result, bonding by powders may be unreliable or insufficient.
Reactive multilayer structure, which were subsequently developed, reduced the problems associated with reactive powder bonding. These structures are comprised of thin coatings that undergo exothermic reactions. See, for example, T. P. Weihs, Handbook of Thin Film Process Technology, Part B, Section F.7, edited by D. A. Glocker and S. I. Shah (IOP Publishing, 1998); U.S. Pat. No. 5,538,795 issued to Barbee, Jr. et al. on Jul. 23, 1996; and U.S. Pat. No. 5,381,944 to D. M. Makowiecki et al. on Jan. 17, 1995. Reactive multilayer structures permit exothermic reactions with more controllable and consistent heat generation. The basic driving force behind such reactions is a reduction in atomic bond energy. When the series of reactive layers is ignited, the distinct layers mix atomically, generating heat locally. This heat ignites adjacent regions of the structure, thereby permitting the reaction to travel the entire length of the structure, generating heat until all the material is reacted.
However, even with this advance, many problems remain. For example, reactive coatings often debond from their substrates upon reaction. This debonding is caused by inherent reactive foil densification during reaction and by non-uniform thermal expansion or contraction during heating and cooling. It significantly weakens the bond in joining applications. More significantly, current reactive multilayer foils yield brittle intermetallic compounds that have limited ductility and therefore can degrade the resultant joints by their presence between the bonded components. Consequently, internal or external stresses can cause catastrophic mechanical failure of the bonds.
In addition to reactive coatings, efforts were made to develop freestanding reactive layers by cold rolling. See L. Battezzatti et al, Acta Materialia, Vol. 47, pp. 1901-1914 (1999). Nixe2x80x94Al multilayer reactive foils were formed by cold-rolling bilayer sheets of Ni and Al, followed by repeated manual folding and repeated cold rolling. After the first bilayer strip was rolled to half its original thickness, it was folded once to regain its original thickness and to double the number of layers. This process was repeated many times.
This fabrication of the rolled foils was time consuming and difficult. The rolling passes require lubricating oil, and the surfaces of the rolled materials must be cleaned after every pass. In addition, the manual folding of sheet stock does not easily lend itself to large-scale production. Starting with a stack of metallic sheets and then rolling and folding a few times would simplify the process. However, when many metal layers are rolled at once, these layers can spring back, causing separation of the layers and degradation of the resulting foil. Such separation also permits undesirable oxidation of interlayer surfaces and impedes unification of the layers by cold welding.
Accordingly, there is a need for improved methods of fabricating reactive multilayer foil.
In accordance with the invention a reactive multilayer foil is fabricated by providing an assembly (stack or multilayer) of reactive layers, inserting the assembly into a jacket, deforming the jacketed assembly to reduce its cross sectional area, flattening the jacketed assembly into a sheet, and then removing the jacket. Advantageously, the assembly is wound into a cylinder before insertion into the jacket, and the jacketed assembly is cooled to a temperature below 100xc2x0 C. and preferably below 25xc2x0 C. during deforming. The resulting multilayer foil is advantageous as a freestanding reactive foil for use in bonding, ignition or propulsion.